The present invention is concerned with the problem of providing a flexible packaging laminate which can be used in sheet form or in the form of a bag or other flexible container for heating articles in a microwave oven. Many products have been proposed for browning, searing or otherwise heating the surface of an article within a microwave oven. These prior products can be divided into three groups: rigid; semi-flexible or having limited flexibility; and flexible. Rigid heating containers are exemplified by the following U.S. Pat. Nos.: 4,266,108; 4,184,061; 4,450,334; 4,398,077; 4,166,208; 4,158,760; and 4,320,274. Among these, U.S. Pat. No. 4,266,108 describes a reflective metal plate such as an aluminum plate to which a layer of lossy materials, e.g. magnetic oxides known as ferrites, have been applied. These materials are bonded to the reflective metal plate which can be aluminum by means of an ntermediate layer containing a binder on an air gap. U.S. Pat. No. 4,184,061 describes a glass ceramic browning vessel with a metallic oxide coating on its lower surface. U.S. Pat. No. 4,450,334 is similar, except that in this cae a plastic layer containing a ferrite is applied to the bottom surface of an aluminum dish. In U.S. Pat. No. 4,398,077 a ceramic or glass dish is described having a resistive film 14 of tin oxide applied to its lower surface. Both of U.S. Pat. Nos. 4,166,208 and 4,158,760 describe conical containers formed from plastic. The lower end of each cone is in contact with the support member such as a block of plastic which is made lossy by the inclusion of metal or carbon particles. U.S. Pat. No. 4,320,270 describes a cooking utensil in the form of a dielectric dish, e.g., glass or plastic, having a metal layer extending through it.
Among references describing flexible packaging materials is U.S. Pat. No. 4,190,757 which describes a supporting substance in the form of aluminum foil which may be as little as one mil in thickness to which a paint-like layer of a ferrite or other lossy material is applied as a coating. For example, a wet mixture of taconite, sand and sodium silicate are blended and applied by brushing and rolling the composition onto a sheet of three mil aluminum to a thickness of 0.03 inches. The resulting laminate is fairly thick (over 30 mils in thickness) and is difficult to handle with automated roll stands and other equipment used for winding, rolling, cutting, transferring and forming sheet material into packages. Moreover, the laminate is heavy and the ferrite coating sometimes tends to flake off when the underlying aluminum sheet is bent or flexed.
U.S. Pat. Nos. 4,267,420 and 4,230,924 describe thin flexible polyester films or laminates of polyester and polyethylene to which a thin semiconducting coating is applied. This coating is typically aluminum which is evaporated onto the plastic film. In developing the present invention, films of this kind were tested experimentally. However, an important problem developed which rendered the patented sheets deficient in some applications. It was noted that a sheet or wrapper often extended away from the surface of the item being heated. The surface of the item being heated may also be irregular so that parts of the film do not conform to it. Moreover, the film often extended into portions of a package where the food product does not have access, e.g. bags or wrappers having a crimped end that did not contact the food. It was discovered that the portions which extended beyond the edges of the item being heated were especially subject to damage. Thus, after a few seconds of heating portions of the film shrank to 1/3 of its original size or less and became melted in the process.
In one test, six cylindrical frozen fishsticks and six frozen chicken patties were each wrapped in a polyester film having a semiconducting evaporated aluminum coating of the type described in the Brastad patents. Each item was heated separately in a 625 watt Kenmore consumer type microwave oven. While a certain amount of surface crisping did take place, the unsupported portions of the film that were not in direct contact with the food shriveled and disintegrated. Unless the film was pressed against the surface it would not remain intact. The portion of the sheet material between the food and the underlying dish remained in one piece, but the tope and sides were particularly susceptible to damage. Especially where there were irregularities in the food, those portions of the film not in contact with the food would burn through. In the case of the chicken patties about 40% of the film became disintegrated and became perforated with holes or otherwise melted, pulling back on itself as it shriveled up after 1.5 minutes of heating at which time the heating was discontinued. It was noticed that the patties were not completely heated and the film did not appear to produce surface browning of the food. Performance was judged unsatisfactory and commercially unacceptable. Moreover, the film did not pull away from the food in a predictable way so that the kind of design changes that might be needed to correct the problem were not apparent.
In view of these and other deficiencies in the prior art it is the general objective of the present invention to provide an improved flexible sheet for packaging purposes and for producing heat in a microwave oven with the following characteristics and advantages:
(a) the ability to absorb microwave energy and transfer the absorbed energy to products in a microwave oven during a heating process without shrinking, burning, shriveling or disintegrating;
(b) the ability to perform satisfactorily although portions of the sheet extend away from and out of heat transfer relationship with the product being heated;
(c) sufficient flexibility to be wound, unwound, transferred either as cut sheets or a continuous film or formed into package structures such as bags and the like on conventional processing and gluing equipment used for packaging films and paper;
(d) the ability to carry out heating in certain selected areas and not in others;
(e) the provision for handling and transferring a film capable of heating products as a continuous uninterrupted strip or web wherein only certain portions of the web perform a heating function,
(f) the provision of cut sheets of a microwave absorbing dimensionally unstable film are located at spaced locations and a provision is made for rendering them dimensionally stable; and
(g) the provision of areas of any selected shape such as rectangular, triangular, hexagonal, circular areas, etc., are capable of heating while other surrounding areas of a flexible sheet have no heating function.
These and other more detailed and specific objects and advantages of the invention will become apparent in view of the following detailed description and accompanying drawings which set forth by way of example certain illustrative embodiments of the invention.